Ultrawideband transverse electromagnetic mode horn transmitter and antenna

ABSTRACT

An ultrawideband transverse electromagnetic mode horn antenna for use at high voltages, comprises a pulse generator (1) and two transmission horns (12, 18) containing different dielectric media. The interface (30) between the dielectric media is configured so that a signal from the generator is incident on the interface at an angle substantially equal to the Brewster angle, thereby maintaining a good impedance match across the interface. A further advantage of the arrangement is that the TEM wavefront is preserved through the antenna section allowing operation at fast pulse risetime (less than 200 ps) for short duration (several ns) at high voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an ultrawideband antenna incorporating a pulsegenerator for use in transverse electromagnetic mode (TEM), particularlyfor use at high voltages.

2. Discussion of Prior Art

There is a desire to develop antenna capable of operating with pulses ofincreasingly high voltage, short duration and rapid pulse risetime andrepetition rate, for example for application in wide bandwidth highdefinition radar systems. The transmission line geometry of such anantenna required to ensure good transmission of pulse energy is aprincipal area of concern, having associated problems which areexacerbated under higher voltage and rapid pulse risetime conditions.

In these antennae, the dielectric medium which contains the generatorand the dielectric medium from which the signal is radiated away fromthe antenna are likely to be different, the former for example adielectric polymer or transformer oil and the latter for example air orother suitable gas. Problems are encountered where the pulse signaloutput from the generator makes the transition between the dielectricmedia. To minimize degradation of the signal by reflection at aninterface it is desirable to minimize impedance variation across it. Thetwo media will possess different dielectric properties however, so thata transmission line having a continuous geometry at the interface willnecessarily produce impedance discontinuities, so that reflection willoccur. Impedance matching for a normally incident signal can be achievedby incorporating geometric discontinuities at the interface but this canalso be expected to produce degradation of pulse quality. A furtherimportant consideration in determining geometry arises from the need tomaintain an insulating condition in the transmission line to avoidbreakdown. Voltage holdoff may reach 30 kV/cm in air at atmosphericpressure which will determine the minimum plate separation for an airfilled transmission line at a given voltage. These factors place limitson the voltage or necessitate use of a pulse having a longer risetime.

SUMMARY OF THE INVENTION

The present invention aims to provide an antenna which offers goodimpedance matching at the interface of the two dielectric media so as topreserve a rapid risetime at high voltage operation.

Thus according to the present invention there is provided an antennacomprising an electromagnetic pulse generator, a first transverseelectromagnetic mode transmission line containing a first dielectricmedium, and a second transverse electromagnetic mode transmission linecontaining a second dielectric medium, serially connected so as toenable transmission of a signal from the generator to the secondtransmission line, wherein the first transmission line incorporates atransition element providing an interface between the first and seconddielectric media which is so configured that a signal from the generatoris incident on the interface at an angle substantially equal to theBrewster Angle.

It will be appreciated that for a given pair of homogeneous dielectricmaterials the Brewster Angle represents the angle at which a plane waveincident on a planar dielectric interface with the magnetic field in adirection parallel to the plane of the interface will undergo noreflection. For a wave passing from medium 2 of higher refractive indexand permittivity ε2 to medium 1 of lower refractive index andpermittivity ε1 the angles Ψ1 and Ψ2 are given by

    tan Ψ2=√(ε2/ε1) and tanΨ1=√(ε1/ε2)

    Ψ1+Ψ2=π/2.

For a polymethylmethacrylate to air transition, for example, theseangles are Ψ2=58.7° and Ψ1=31.3°. In order to use this principle tomatch the impedances of two TEM horns which are located on either sideof the media interface and meet along it consider firstly two striplinesin media 1 and 2 of heights H1 and H2 of equal width W. Then ##EQU1##This implies that the impedance of the striplines (given approximatelyby Z_(O) (H1/W) and Z_(O) √(ε1/ε2) (H2/W) when H1,H2<<W) are matchedacross the interface (ignoring any effects arising from the finite widthof the stripline). The striplines can be replaced by TEM horn elementsprovided their elevation and taper is shallow to ensure that thewavefront arriving at the interface is nearly planar ; small deviationsfrom exact planarity will produce small reflections.

Use of the Brewster Angle concept in this way provides a method ofconfiguring the dielectric transition which maintains a good impedancematch across the transition. At the same time geometric discontinuitiesat the interface which inevitably arise with impedance matching fornormal incidence are reduced. The TEM wavefront structure is preservedthrough the antenna section and allows operation on at fast pulserisetime (less than 200 ps) for short duration (several ns) at highvoltage.

The second dielectric medium is often conveniently gaseous. For manyapplications it is usefully the same as the medium into which theantenna is to be used to broadcast a signal, and hence for mostoperations the second dielectric medium is conveniently air. However,for some applications it is desirable to use as the second dielectricmedium a gaseous dielectric with a higher breakdown potential than air,to allow operation at higher voltage at a given geometry. A range ofsuch gases. for example SF₆, can be considered for this purpose.

The interface between the first dielectric medium and the seconddielectric medium must be accurately configured, and this isconveniently achieved when the first dielectric medium possessesreasonable rigidity and machinability. Polymeric materials such aspolymethylmethacrylate (PMMA), polystyrene. polytetrafluoroethylene(PTFE) and the like are suitable for this purpose. Alternatively, thefirst dielectric medium may comprise a liquid dielectric such astransformer oil in combination with an interface element of rigidmaterial and substantially similar dielectric constant to the liquid toconstrain the liquid and provide an accurate interface.

The invention is particularly appropriate to operation with pulses athigh voltage and rapid risetime. The electromagnetic pulse generator istherefore preferably capable in use of generating a pulse at a voltagegreater than 30 kV, more preferably greater than 60 kV, most preferablygreater than 100 kV, and is preferably capable in use of generating apulse having a risetime of less than 200 ps, more preferably less than120 ps. The pulses are also preferably of short duration (of the orderof a few nanoseconds). To achieve signals within these parameters, theelectromagnetic pulse generator preferably includes signal sharpeningmeans such as a spark gap or ferrite sharpening lines.

The first and second transmission lines may in a simple embodiment eachcomprise parallel conducting plate transmission lines, but much improvedperformance can be obtained when one or both of the transmission linescomprise a transverse electromagnetic mode horn. The Brewster Angleconcept applied herein requires a plane wave incident on a planarboundary with a magnetic component parallel to the planar boundary. Itis apparent therefore that the wavefront must maintain characteristicsapproximating to planarity as it passes through the transmission means,so that the angular separation between upper and lower conductors of thehorn and the apex angle must be sufficiently small to maintainapproximate planarity of the wavefront. Within these constraints theradiated field strength from the antenna can be maximized by reducingimpedance mismatch between the aperture of the second horn which servesto radiate the signal from the antenna, and the medium into which thesignal is radiated (usually this will mean matching up with theimpedance of air/free space). This must be done whilst retainingimpedance matching at the dielectric interface, so the second horn ispreferably profiled such that its impedance increases with distance fromthe interface towards an aperture so as to be substantially matched atthe aperture to the impedance of the medium into which the hornradiates. In addition, the second horn may be resistively loaded inorder to attenuate currents reflected from the antenna aperture, whichcurrents can cause undesirable features in the radiated pulse.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only withreference to FIGS. 1 to 5 in which;

FIG. 1 is a plan view of a ground plane antenna according to anembodiment of the invention;

FIG. 2 is a longitudinal cross section of the antenna of FIG. 1;

FIG. 3 is a longitudinal cross section of a modified ground planeantenna based on the embodiment of FIGS. 1 and 2;

FIG. 4 is a plan view of a free field antenna according to analternative embodiment of the invention;

FIG. 5 is a longitudinal cross section of the antenna of FIG. 4.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

In FIGS. 1 and 2 there is provided a ground plane antenna designed tooperate at 50 ohm impedance. An electromagnetic pulse generator 1 ismade up of a spark gap generator incorporating a pulser 3 and a sparkgap 2, and a parallel plate transmission line feed 4 which comprises agrounded plate 6 and parallel upper plate 8 having PMMA as a dielectric10. For 50 ohm operation with a PMMA dielectric the transmission linefeed 4 requires a width/height ratio (W3/H3) of approximately 2.4. Thefeed enables the sharpened pulse signal output from the spark gap 2 tobe passed to a PMMA filled TEM horn 12. The spark gap may alternativelybe configured so that it lies across and shorts the plates 6 and 8 sothat the rear of the applied pulse is sharpened rather than the front aswould otherwise be the case. The spark gap medium may be solid, gaseousor liquid.

The horn 12 has a grounded plate 14 and an upper plate 16 maintained atan angle of elevation thereto θ1 which is kept shallow to ensure thatthe wavefront maintains characteristics approximating to planarity whichare necessary for the Brewster Angle principle to be applied. In thisparticular embodiment 8.5° has been found an optimum compromise for theangle θ1 between the advantages which a horn antenna offers over asimple stripline and the need to maintain an approximately planarwavefront. The plates 14, 16 are configured to produce a horn with anapex angle θ2 of 19.5°.

The second transmission line, from which the signal is radiated awayfrom the antenna, is an air filled TEM horn 18 comprising a ground plate20 and upper plate 22 configured to give a virtual apex angle θ3 of 40°.The upper plate 22 is maintained at an angle of elevation of 8.5°relative to the grounded plate 20.

Between the air filled horn 18 and the PMMA filled horn 12 there isprovided a transition element 24. The transition element 24 has an upperplate 25 with the grounded plate 20 serving as its lower plate, partlycontaining a PMMA dielectric which is continuous with the PMMAdielectric in the horn 12 and parallel plate line feed 4, and alsocontaining air 28 which is continuous with the air in the horn 18. Theinterface between the two media 30 is shaped to ensure the wavefront isincident at the Brewster Angle for a PMMA air transition, which requiresangles Ψ1 and Ψ2 to be 31.3° and 58.7° respectively The heights H2, H1,and hence the other dimensions of the antenna, are governed by the needto avoid breakdown of the signal and hence are determined by theoperating voltage. For 30 kV operation H2=58 mm and H1=35 mm are foundacceptable, and this enables the length of the PMMA horn to be keptreasonably short (30 cm with this geometry) so that dispersion of thesignal is kept sufficiently low to allow operation with pulse risetimesof the order of 120 ps. For higher voltage operation H2 and H1 may beincreased, but the corresponding increase in length of the PMMA filledhorn will increase the minimum usable pulse risetime, and it maytherefore be preferable to consider other modifications, such as asulphur hexafluoride jacket in the transition region.

For ease of manufacture the conducting plates 6, 8, 14, 16, 20, 22 and25 are constructed from aluminium, but alternatives will readily suggestthemselves as appropriate to those skilled in the art.

FIG. 3 illustrates a modification of the antenna of FIGS. 1 and 2, andlike numerals are used to designate like components where appropriate.In this embodiment there is provided a stripline feed 4, PMMA filledhorn 12, and transition element 24 of equivalent design to the above.However, this embodiment sharpens the signal by means of a ferritesharpening line 27. A signal from a pulser (not shown) is passed to acoaxial line comprising a pair of coaxial conductors 29 with a length offerrite material 32 included in the core to effect sharpening. Thesharpened signal passes via a coaxial/stripline converter 33 to thestripline 4, and thence to the first TEM horn 12 as in the earlierembodiment. A spark gap may also be configured as coaxial, in which casea similar coaxial to stripline converter 33 is required.

As the antennae in these embodiments are designed for 50 ohm operationthere is appreciable impedance mismatch with free space when the signalreaches the air filled horn aperture of the earlier embodiment. Tomitigate this, the embodiment illustrated in FIG. 3 includes analternative air filled horn 19, having an upper plate 23 which is nolonger planar but is instead divergently curved away from the groundplate 21 so that the height to width ratio, and hence the impedance,increases between dielectric interface 30 and aperture 31. The impedanceat the aperture thereby more nearly coincides with that of free spaceallowing radiated field strength to be maximized.

In addition, undesirable reflections from a mismatched antenna aperturemay be reduced by applying a resistive loading across the ground plate20 and upper plate 22 in order to provide a continuously increasingresistive profile between the dielectric interface 30 and aperture 31.This can be achieved by applying a resistive coating or chip resistorsto one or both of the plates to approximate such a load or a resistivetermination to ground could be applied to the ends of the antenna. Forexample, two 100 ohm resistors 39 connected in parallel between theground plate 20 and the upper plate 22 at their aperture ends wouldmatch 50 ohm assumed impedance of the antenna.

In FIGS. 4 and 5 there is provided a free field antenna configuredaccording to similar principles to the ground plane antenna in FIGS. 1and 2.

A pulse generator 35 is provided in which pulse sharpening is effectedby means of a spark gap 37. A parallel plate transmission line feed 34which comprises parallel conducting plates 36 containing a PMMAdielectric 38 is used to transmit the fast risetime short duration pulseto a PMMA filled TEM horn 40. The horn 40 has a pair of aluminium plates42 configured to have an angular separation of 8° (that is, the anglesθ2 are 4°) and the plates are flared at an angle of 12.75° to produce anapex angle θ5 of 25.5°. These are again chosen to ensure that thewavefront maintains characteristics approximating to planarity. Thesecond transmission means again comprises an air filled TEM horn 44 madeup of a pair of aluminium plates 46, 47 configured to the same 8°angular separation and flared at 23° to produce a virtual apex angle θ6of 46°.

The transition element 48 consists of an upper aluminium plate 50 whichlies in a plane parallel to that of the transmission line 34 and a loweraluminium plate 52. The PMMA dielectric in the transition zone 54 isshaped to ensure an angle of incidence at the interface 56 with aircorresponding to the Brewster angle, so that Ψ3 is 58.7° and the lowerplate 52 is at an angle Ψ4 to the interface 56 of 31.3°. The height towidth cross-sectional ratio of the upper antenna arm 46 must beapproximately equal to that of a notional 50 ohm air filled stripline tominimize any mismatch. This requires a slight flaring of the upper plate50 in the transition element over and above the 12.75° flaring of theplates 42. Similar considerations lead to a flaring angle for the lowerplate 52 in the transition element which is less than 12.75°. The anglesinvolved do not create major discontinuities within this region, so anymismatch will be small, of the order of 10% or less.

Individual antenna elements may be assembled as an array in order toincrease the radiated power.

We claim:
 1. An antenna for transmitting an ultrawidebandelectromagnetic pulse from an electromagnetic pulse generator, saidantenna comprising:a first transverse electromagnetic mode transmissionline containing a first dielectric medium, and a second transverseelectromagnetic mode transmission line containing a second dielectricmedium, serially connected so as to enable transmission of a signal fromsaid first dielectric medium to the second transmission line, and atransition element providing an interface between the first and seconddielectric media, said first and second transmission lines and saidtransition element configured such that a signal from the firsttransmission line is incident on the interface at an angle substantiallyequal to the Brewster Angle.
 2. Antenna according to claim 1 wherein thesecond dielectric medium is air.
 3. Antenna according to claim 1 whereinthe second dielectric medium is a gaseous dielectric with a higherbreakdown potential than air.
 4. Antenna according to claim 1 whereinthe first dielectric medium is selected from the group comprisingpolymethylmethacrylate, polystyrene, and polytetrafluoroethylene. 5.Antenna according to claim 1 wherein the first transmission linecomprises a first transverse electromagnetic mode horn.
 6. Antennaaccording to claim 1 wherein the second transmission line comprises asecond transverse electromagnetic mode horn.
 7. Antenna according toclaim 6 wherein the second horn is profiled such that its impedanceincreases with distance from the interface towards an aperture so as tobe substantially matched at the aperture to the impedance of the mediuminto which the horn radiates.
 8. Antenna according to claim 6 whereinthe second horn is resistively loaded such that its resistive profileincreases with distance from the interface towards an aperture so as tobe substantially matched at the aperture to the impedance of the mediuminto which the horn radiates.
 9. Antenna according to claim 1, whereinsaid first transmission line comprises a grounded plate and an upperplate, said plates separated by said first dielectric medium, said upperplate having an angle of elevation θ1 with respect to said lower plate,said angle θ1 maintained shallow enough such that a wavefront of saidpulse in said first transmission line is substantially planar. 10.Antenna according to claim 9, wherein said angle θ1 is substantially 8.5degrees.
 11. Antenna according to claim 1, wherein said firsttransmission line comprises a grounded plate and an upper plate, saidplates separated by said first dielectric medium, said secondtransmission line comprises a grounded plate and an upper plate, saidplates separated by said second dielectric medium, said first and secondtransmission lines upper and lower plates having an angle of elevationθ1 with respect to said first and second transmission lines lowerplates, said angle θ1 maintained shallow enough such that a wavefront ofsaid pulse in said first transmission line is substantially planar. 12.Antenna according to claim 1, wherein said transition elementcomprises:an upper plate section; and a dielectric medium, saidtransition element dielectric medium the same as said first dielectricmedium, and using said second transmission line lower plate section,said transition element upper plate section parallel with said firsttransmission line lower plate, said transition element dielectriccontinuous with said first dielectric medium and having a planarinterface with said second dielectric medium, said second transmissionline lower plate section disposed at a first Brewster Angle Ψ1 withrespect to said planar interface and said transition element upper platedisposed at a second Brewster Angle Ψ2 with respect to said planarinterface.
 13. Antenna according to claim 12, wherein said firstdielectric medium is polymethylmethacrylate, said second dielectricmedium is air, said first Brewster Angle Ψ1 is substantially 31.3degrees and said second Brewster Angle Ψ2 is substantially 58.7 degrees.14. A transmitter for transmitting an ultrawideband electromagneticsignal, said transmitter comprising:an electromagnetic pulse generatorfor generating an electromagnetic signal; a first transverseelectromagnetic mode transmission line containing a first dielectricmedium for receiving said electromagnetic signal from said generator; asecond transverse electromagnetic mode transmission line containing asecond dielectric medium, serially connected so as to enabletransmission of said signal from said first dielectric medium to thesecond transmission line; and a transition element providing aninterface between the first and second dielectric media which is soconfigured that said signal from the generator is incident on theinterface at an angle substantially equal to the Brewster Angle. 15.Antenna according to claim 14 wherein the electromagnetic pulsegenerator is capable of generating a pulse at a voltage greater than 30kV.
 16. Antenna according to claim 15 wherein the electromagnetic pulsegenerator is capable of generating a pulse at a voltage greater than 60kV.
 17. Antenna according to claim 16 wherein the electromagnetic pulsegenerator is capable of generating a pulse at a voltage greater than 100kV.
 18. Antenna according to claim 14 wherein the electromagnetic pulsegenerator is capable of generating a pulse having a risetime of lessthan 200 ps.
 19. Antenna according to claim 18 wherein theelectromagnetic pulse generator is capable of generating a pulse havinga risetime of less than 120 ps.
 20. Antenna according to claim 14wherein the electromagnetic pulse generator includes signal sharpeningmeans.
 21. Antenna according to claim 20 wherein the signal sharpeningmeans comprises a spark gap.
 22. Antenna according to claim 20 whereinthe signal sharpening means comprises ferrite sharpening lines.